This invention relates to a method and apparatus for reducing the content of nitrogen oxides from internal-combustion engines. More specifically, the invention relates to a system for cleaning up lean burnt exhaust that is applicable to a "lean burnt engine" which uses a dilute air-fuel mixture with a view to improving fuel economy, as well as a diesel engine, a hydrogen engine and a Stirling engine (the last-mentioned is an external-combustion engine) and which is capable of effectively reducing and cleaning up nitrogen oxides (hereunder sometimes abbreviated as NOx) in the exhaust irrespective of its concentration of oxygen gas (hereunder sometimes abbreviated as O.sub.2) without impairing the good fuel economy of those engines.
Conventionally, three methods have been proposed as means of reducing the content of NOx in the exhaust from internal-combustion engines, principally piston engines and they are:
(1) use of a three-way catalyst;
(2) use of an ultra-lean air/fuel ratio; and
(3) use of a lean NOx catalyst.
These methods have their own disadvantages. The first method requires that the air and fuel mixture to be supplied to the engine should be precisely controlled to the stoichiometric air/fuel ratio (ca. 14.5:1). If the fuel is leaner than the stoichiometric ratio, the content of NOx is not reduced. However, it is known that fuel economy is promoted by operating the engine on the fuel-lean side as shown in FIG. 2. The second method is intended to achieve both lower NOx content and better fuel economy with a "lean burnt engine". However, if one wants to use an air/fuel ratio that is capable of satisfactory reduction in the NOx content, the fuel-air mixture approaches the misfire limit of combustion and this deteriorates not only the fuel economy of the engine but also the drivability of the vehicle. In order to avoid these problems, methods have been proposed by which the air stream in the cylinder is provided with turbulence or increased in flow rate so that the combustion speed is sufficiently increased to bring the misfire limit to a further fuel-lean side. However, this is not expected to achieve a satisfactory effect because if the misfire limit is brought to a further fuel-lean side (see FIG. 3), the NOx emission will decrease by a smaller degree as indicated by the dashed line. To compensate for these problems of the second method, it has been proposed by the third method that the engine be operated at an air/fuel ratio that is near the point where the fuel consumption is at minimum, which point is somewhat closer to the stoichiometric ratio than the misfire limit. The NOx whose content tends to decrease by an insufficient degree in the second method is cleaned up with a zeolite-based lean NOx catalyst in the third method. This approach can potentially provide a system of better fuel economy. However, the lean NOx catalyst which reduces NOx in the presence of a large amount of O.sub.2 in the exhaust is required to meet strict temperature and other conditions and this presents one major practical problem to be solved in that it is difficult to achieve satisfactory catalytic of NOx while using the catalyst over an extended period (high catalyst durability). Thus, each of the methods so far proposed for achieving satisfactory reduction in NOx content while using an air/fuel ratio that is capable of achieving maximum improvement in the fuel economy of the engine has many problems to be solved before they are put to practical use.